The Republican chairman is stepping aside from leading a congressional investigation of Russian interference in last year’s U.S. presidential election, citing ethics complaints that he mishandled classified information.

The decision by Rep. Devin Nunes of California comes amid partisan turmoil on the House intelligence committee. Democrats have alleged that Nunes, who was on President Donald Trump’s transition team, is too close to the White House and cannot lead an impartial inquiry. In an extraordinary step, the ...